The distribution of macromolecules throughout the body is generally diffusion mediated with macromolecules in the blood diffusing into the tissues across highly fenestrated endothelial cell linings of the capillary vasculature. Free diffusion of macromolecules does not exist in the highly vascularized brain. Brain capillary endothelial cells lack the fenestrations seen in the rest of the circulatory system and have highly specialized endothelial cell tight intracellular junctions. These tight junctions serve to prevent the free diffusion of molecules of greater than 400 kDa from the luminal to the abluminal side of the capillary. In addition, the capillaries contain various transporter systems such as the Organic Anion Transporters (OATS) and Multi-Drug Resistance (MDR) systems that actively establish transportation gradients of molecules that might otherwise diffuse through endothelial cells. The combination of the restrictive barriers prevents the entrance of adventitious agents, including toxins and viruses for example, as well as restricting the diffusion of therapeutic entities. In addition these restrictive blood brain barriers (BBB) effectively block the passive delivery of potentially therapeutic proteins, peptides and small molecules into the brain parenchyma at pharmacologically therapeutic doses.
One successful strategy to achieve transport of therapeutic molecules across the BBB endothelial cells has been to take advantage of receptor mediated transcytosis (RMT). This strategy uses antibodies or molecules that bind specifically to proteins on BBB endothelial cells that are typically involved in the transport of molecules across the BBB endothelial cells. Such antibodies are used as shuttle molecules to deliver attached payloads while undergoing transcytosis across the BBB endothelial cells. Examples of the applications of this technology include the use of antibodies to the transferrin receptor and insulin receptors (Yu et al. 2011. Science Translational Medicine. Volume 3). In these two cases RMT antibodies were fused C-terminally to therapeutic protein domains and have been shown to transport molecules across the BBB. Unfortunately, commonly used RMT targets transferrin and insulin receptors are highly and broadly expressed in numerous tissues. This broad target expression results in a short circulating half-life, that in turn limits the time of exposure to BBB endothelial cells and thereby the dosing of the molecule into the brain. In addition, these antibodies target metabolically critical cellular functions thereby creating a potential safety risk.
Improved targeting moieties that make use of active BBB transport molecules to cross the BBB e.g., binding sites derived from antibody molecules that transmigrate across the BBB, would be of great benefit for the delivery of therapeutics into the brain.